Network Working Group A. DeKok
INTERNET-DRAFT FreeRADIUS
Category: Experimental
<draft-ietf-radext-tcp-transport-08.txt>
Expires: February 20, 2011
1 July 2010RADIUS Over TCPdraft-ietf-radext-tcp-transport-08
Abstract
The Remote Authentication Dial In User Server (RADIUS) Protocol has
until now required the User Datagram Protocol (UDP) as the underlying
transport layer. This document defines RADIUS over the Transmission
Control Protocol (RADIUS/TCP), in order to address handling issues
related to RADIUS over Transport Layer Security (RADIUS/TLS). It
permits TCP to be used as a transport protocol for RADIUS only when a
transport layer such as TLS or IPsec provides confidentialy and
security.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with
the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet-Drafts
as reference material or to cite them other than as "work in
progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
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This Internet-Draft will expire on February 1, 2011
Copyright Notice
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Copyright (c) 2010 IETF Trust and the persons identified as the
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INTERNET-DRAFT RADIUS Over TCP 1 July 20101. Introduction
The RADIUS Protocol is defined in [RFC2865] as using the User
Datagram Protocol (UDP) for the underlying transport layer. While
there are a number of benefits to using UDP as outlined in [RFC2865]
Section 2.4, there are also some limitations:
* Unreliable transport. As a result, systems using RADIUS have to
implement application-layer timers and re-transmissions, as
described in [RFC5080] Section 2.2.1.
* Packet fragmentation. [RFC2865] Section 3 permits RADIUS
packets up to 4096 octets in length. These packets are larger
than the common Internet MTU (576), resulting in fragmentation of
the packets at the IP layer when they are proxied over the
Internet. Transport of fragmented UDP packets appears to be a
poorly tested code path on network devices. Some devices appear
to be incapable of transporting fragmented UDP packets, making it
difficult to deploy RADIUS in a network where those devices are
deployed.
* Connectionless transport. Neither clients nor servers receive
positive statements that a "connection" is down. This information
has to be deduced instead from the absence of a reply to a
request.
* Lack of congestion control. Clients can send arbitrary amounts
of traffic with little or no feedback. This lack of feedback can
result in congestive collapse of the network.
RADIUS has been widely deployed for well over a decade, and continues
to be widely deployed. Experience shows that these issues have been
minor in some use-cases, and problematic in others. For use-cases
such as inter-server proxying, an alternative transport and security
model -- RADIUS/TLS or RADIUS/TLS, as defined in [RADIUS/TLS]. That
document describes the transport implications of running RADIUS/TLS.
The choice of TCP as a transport protocol is largely driven by the
desire to improve the security of RADIUS by using RADIUS/TLS. For
practical reasons, the transport protocol (TCP) is defined separately
from the security mechanism (TLS).
Since "bare" TCP does not provide for confidentiality or enable
negotiation of credible ciphersuites, its use is not appropriate for
inter-server communications where strong security is required. As a
result "bare" TCP transport MUST NOT be used without TLS, IPsec, or
other secure upper layer.
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"Bare" TCP transport MAY, however, be used when another method such
as IPSec [RFC4301] is used to provide additional confidentiality and
security. Should experience show that such deployments are useful,
this specification could be moved to standards track.
1.1. Applicability of Reliable Transport
The intent of this document is to address transport issues related to
RADIUS/TLS [RADIUS/TLS] in inter-server communications scenarios,
such as inter-domain communication between proxies. These situations
benefit from the confidentiality and ciphersuite negotiation that can
be provided by TLS. Since TLS is already widely available within the
operating systems used by proxies, implementation barriers are low.
In scenarios where RADIUS proxies exchange a large volume of packets,
it is likely that there will be sufficient traffic to enable the
congestion window to be widened beyond the minimum value on a long-
term basis, enabling ACK piggy-backing. Through use of an
application-layer watchdog as described in [RFC3539], it is possible
to address the objections to reliable transport described in
[RFC2865] Section 2.4 without substantial watchdog traffic, since
regular traffic is expected in both directions.
In addition, use of RADIUS/TLS has been found to improve operational
performance when used with multi-round trip authentication mechanisms
such as EAP over RADIUS [RFC3579]. In such exchanges, it is typical
for EAP fragmentation to increase the number of round-trips required.
For example, where EAP-TLS authentication [RFC5216] is attempted and
both the EAP peer and server utilize certificate chains of 8KB, as
many as 15 round-trips can be required if RADIUS packets are
restricted to the common Ethernet MTU (1500 octets) for EAP over LAN
(EAPoL) use-cases. Fragmentation of RADIUS/UDP packets is generally
inadvisable due to lack of fragmentation support within intermediate
devices such as filtering routers, firewalls and NATs. However,
since RADIUS/UDP implementations typically do not support MTU
discovery, fragmentation can occur even when the maximum RADIUS/UDP
packet size is restricted to 1500 octets.
These problems disappear if a 4096 application-layer payload can be
used alongside RADIUS/TLS. Since most TCP implementations support
MTU discovery, the TCP MSS is automatically adjusted to account for
the MTU, and the larger congestion window supported by TCP may allow
multiple TCP segments to be sent within a single window. Even those
few TCP stacks which do not perform path MTU discovery can already
support arbitrary payloads.
Where the MTU for EAP packets is large, RADIUS/EAP traffic required
for an EAP-TLS authentication with 8KB certificate chains may be
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reduced to 7 round-trips or less, resulting in substantially reduced
authentication times.
In addition, experience indicates that EAP sessions transported over
RADIUS/TLS are less likely to abort unsuccessfully. Historically,
RADIUS over UDP implementations have exhibited poor retransmission
behavior. Some implementations retransmit packets, others do not,
and others send new packets rather then performing retransmission.
Some implementations are incapable of detecting EAP retransmissions,
and will instead treat the retransmitted packet as an error. As a
result, within RADIUS/UDP implementations, retransmissions have a
high likeilhood of causing an EAP authentication session to fail.
For a system with a million logins a day running EAP-TLS mutual
authentication with 15 round-trips, and having a packet loss
probability of P=0.01%, we expect that 0.3% of connections will
experience at least one lost packet. That is, 3,000 user sessions
each day will experience authentication failure. This is an
unacceptable failure rate for a mass-market network service.
Using a reliable transport method such as TCP means that RADIUS
implementations can remove all application-layer retransmissions, and
instead rely on the Operating System (OS) kernel's well-tested TCP
transport to ensure Path MTU discovery and reliable delivery. Modern
TCP implementations also implement anti-spoofing provisions, which is
more difficult to do in a UDP application.
In contrast, use of TCP as a transport between a NAS and a RADIUS
server is usually a poor fit. As noted in [RFC3539] Section 2.1, for
systems originating low numbers of RADIUS request packets, inter-
packet spacing is often larger than the packet RTT, meaning that, the
congestion window will typically stay below the minimum value on a
long-term basis. The result is an increase in packets due to ACKs as
compared to UDP, without a corresponding set of benefits. In
addition, the lack of substantial traffic implies the need for
additional watchdog traffic to confirm reachability.
As a result, the objections to reliable transport indicated in
[RFC2865] Section 2.4 continue to apply to NAS-RADIUS server
communications and UDP SHOULD continue to be used as the transport
protocol in this scenario. In addition, it is recommended that
implementations of "RADIUS Dynamic AUthorization Extensions"
[RFC5176] SHOULD continue to utilize UDP transport, since the volume
of dynamic authorization traffic is usually expected to be small.
1.2. Terminology
This document uses the following terms:
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RADIUS client
A device that provides an access service for a user to a network.
Also referred to as a Network Access Server, or NAS.
RADIUS server
A device that provides one or more of authentication,
authorization, and/or accounting (AAA) services to a NAS.
RADIUS proxy
A RADIUS proxy acts as a RADIUS server to the NAS, and a RADIUS
client to the RADIUS server.
RADIUS request packet
A packet originated by a RADIUS client to a RADIUS server. e.g.
Access-Request, Accounting-Request, CoA-Request, or Disconnect-
Request.
RADIUS response packet
A packet sent by a RADIUS server to a RADIUS client, in response to
a RADIUS request packet. e.g. Access-Accept, Access-Reject,
Access-Challenge, Accounting-Response, CoA-ACK, etc.
RADIUS/UDP
RADIUS over UDP, as defined in [RFC2865].
RADIUS/TCP
RADIUS over TCP, as defined in this document.
RADIUS/UDP
RADIUS over TLS,, as defined in [RADIUS/TLS].
1.3. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Changes to RADIUS
RADIUS/TCP involves sending RADIUS application messages over a TCP
connection. In the sections that follow, we discuss the implications
for the RADIUS packet format (Section 2.1), port usage (Section 2.2),
RADIUS MIBs (Section 2.3) and RADIUS proxies (Section 2.5). TCP-
specific issues are discussed in Section 2.6.
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The RADIUS packet format is unchanged from [RFC2865], [RFC2866], and
[RFC5176]. Specifically, all of the following portions of RADIUS
MUST be unchanged when using RADIUS/TCP:
* Packet format
* Permitted codes
* Request Authenticator calculation
* Response Authenticator calculation
* Minimum packet length
* Maximum packet length
* Attribute format
* Vendor-Specific Attribute (VSA) format
* Permitted data types
* Calculations of dynamic attributes such as CHAP-Challenge,
or Message-Authenticator.
* Calculation of "encrypted" attributes such as Tunnel-Password.
The use of TLS transport does not change the calculation of security-
related fields (such as the Response-Authenticator) in RADIUS
[RFC2865] or RADIUS Dynamic Authorization [RFC5176]. Calculation of
attributes such as User-Password [RFC2865] or Message-Authenticator
[RFC3579] also does not change.
Clients and servers MUST be able to store and manage shared secrets
based on the key described above, of (IP address, port, transport
protocol).
The changes to RADIUS implementations required to implement this
specification are largely limited to the portions that send and
receive packets on the network.
2.2. Assigned Ports for RADIUS/TCP
IANA has already assigned TCP ports for RADIUS and RADIUS/TLS
transport, as outlined below:
* radius 1812/tcp
* radius-acct 1813/tcp
* radius-dynauth 3799/tcp
* radsec 2083/tcp
Since these ports are unused by existing RADIUS implementations, the
assigned values MUST be used as the default ports for RADIUS over
TCP.
The early deployment of RADIUS was done using UDP port number 1645,
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which conflicts with the "datametrics" service. Implementations
using RADIUS/TCP MUST NOT use TCP ports 1645 or 1646 as the default
ports for this specification.
The "radsec" port (2083/tcp) SHOULD be used as the default port for
RADIUS/TLS. The "radius" port (1812/tcp) SHOULD NOT be used for
RADIUS/TLS.
2.3. Management Information Base (MIB)
The MIB Module definitions in [RFC4668], [RFC4669], [RFC4670],
[RFC4671], [RFC4672], and [RFC4673] are intended to be used for
RADIUS over UDP. As such, they do not support RADIUS/TCP, and will
need to be updated in the future. Implementations of RADIUS/TCP
SHOULD NOT re-use these MIB Modules to perform statistics counting
for RADIUS/TCP connections.
2.4. Detecting Live Servers
As RADIUS is a "hop by hop" protocol, a RADIUS proxy shields the
client from any information about downstream servers. While the
client may be able to deduce the operational state of the local
server (i.e. proxy), it cannot make any determination about the
operational state of the downstream servers.
Within RADIUS as defined in [RFC2865], proxies typically only forward
traffic between the NAS and RADIUS server, and do not generate their
own responses. As a result, when a NAS does not receive a response
to a request, this could be the result of packet loss between the NAS
and proxy, a problem on the proxy, loss between the RADIUS proxy and
server, or a problem with the server.
When UDP is used as a transport protocol, the absence of a reply can
cause a client to deduce (incorrectly) that the proxy is unavailable.
The client could then fail over to another server, or conclude that
no "live" servers are available (OKAY state in [RFC3539] Appendix A).
This situation is made even worse when requests are sent through a
proxy to multiple destinations. Failures in one destination may
result in service outages for other destinations, if the client
erroneously believes that the proxy is unresponsive.
For RADIUS/TLS, it is RECOMMENDED that implementations utilize the
existence of a TCP connection along with the application layer
watchdog defined in [RFC3539] Section 3.4 to determine that the
server is "live".
RADIUS clients using RADIUS/TCP MUST mark a connection DOWN if the
network stack indicates that the connection is no longer active. If
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the network stack indicates that connection is still active, Clients
MUST NOT decide that it is down until the application layer watchdog
algorithm has marked it DOWN ([RFC3539] Appendix A). RADIUS clients
using RADIUS/TCP MUST NOT decide that a RADIUS server is unresponsive
until all TCP connections to it have been marked DOWN.
The above requirements do not forbid the practice of a client pro-
actively closing connections, or marking a server as DOWN due to an
administrative decision.
2.5. Congestion Control Issues
Additional issues with RADIUS proxies involve transport protocol
changes where the proxy receives packets on one transport protocol,
and forwards them on a different transport protocol. There are
several situations in which the law of "conservation of packets"
could be violated on an end-to-end basis (e.g. where more packets
could enter the system than could leave it on a short-term basis):
* Where TCP is used between proxies, it is possible that the
bandwidth consumed by incoming UDP packets destined to a given
upstream server could exceed the sending rate of a single TCP
connection to that server, based on the window size/RTT estimate.
* It is possible for the incoming rate of TCP packets destined to
a given realm to exceed the UDP throughput achievable using the
transport guidelines established in [RFC5080]. This could happen,
for example, where the TCP window between proxies has opened, but
packet loss is being experienced on the UDP leg, so that the
effective congestion window on the UDP side is 1.
Intrinsically, proxy systems operate with multiple control loops
instead of one end-to-end loop, and so are less stable. This is true
even for TCP-TCP proxies. As discussed in [RFC3539], the only way to
achieve stability equivalent to a single TCP connection is to mimic
the end-to-end behavior of a single TCP connection. This typically
is not achievable with an application-layer RADIUS implementation,
regardless of transport.
2.6. TCP Specific Issues
The guidelines defined in [RFC3539] for implementing a AAA protocol
over reliable transport are applicable to RADIUS/TLS.
The Application Layer Watchdog defined in [RFC3539] Section 3.4 MUST
be used. The Status-Server packet [STATUS] MUST be used as the
application layer watchdog message. Implementations MUST reserve one
RADIUS ID per connection for the application layer watchdog message.
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This restriction is described further below in Section 2.6.4.
RADIUS/TLS Implementations MUST support receiving RADIUS packets over
both UDP and TLS transports originating from the same endpoint.
RADIUS packets received over UDP MUST be replied to over UDP; RADIUS
packets received over TLS MUST be replied to over TLS. That is,
RADIUS clients and servers MUST be treated as unique based on a key
of the three-tuple (IP address, port, transport protocol).
Implementations MUST permit different shared secrets to be used for
UDP and TCP connections to the same destination IP address and
numerical port.
This requirement does not forbid the traditional practice of using
primary and secondary servers in a fail-over relationship. Instead,
it requires that two services sharing an IP address and numerical
port, but differing in transport protocol, MUST be treated as
independent services for the purpose of fail-over, load-balancing,
etc.
Whenever the underlying network stack permits the use of TCP
keepalive socket options, their use is RECOMMENDED.
2.6.1. Duplicates and Retransmissions
As TCP is a reliable transport, implementations MUST NOT retransmit
RADIUS request packets over a given TCP connection. Similarly, if
there is no response to a RADIUS packet over one TCP connection,
implementations MUST NOT retransmit that packet over a different TCP
connection to the same destination IP address and port, while the
first connection is in the OKAY state ([RFC3539] Appendix A).
However, if the TCP connection is broken or closed, retransmissions
over new connections are permissible. RADIUS request packets that
have not yet received a response MAY be transmitted by a RADIUS
client over a new TCP connection. As this procedure involves using a
new source port, the ID of the packet MAY change. If the ID changes,
any security attributes such as Message-Authenticator MUST be
recalculated.
If a TCP connection is broken or closed, any cached RADIUS response
packets ([RFC5080] Section 2.2.2) associated with that connection
MUST be discarded. A RADIUS server SHOULD stop processing of any
requests associated with that TCP connection. No response to these
requests can be sent over the TCP connection, so any further
processing is pointless. This requirement applies not only to RADIUS
servers, but also to proxies. When a client's connection to a proxy
server is closed, there may be responses from a home server that were
supposed to be sent by the proxy back over that connection to the
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client. Since the client connection is closed, those responses from
the home server to the proxy server SHOULD be silently discarded by
the proxy.
Despite the above discussion, RADIUS servers SHOULD still perform
duplicate detection on received packets, as described in [RFC5080]
Section 2.2.2. This detection can prevent duplicate processing of
packets from non-conformant clients.
RADIUS packets SHOULD NOT be re-transmitted to the same destination
IP and numerical port, but over a different transport protocol.
There is no guarantee in RADIUS that the two ports are in any way
related. This requirement does not, however, forbid the practice of
putting multiple servers into a fail-over or load-balancing pool. In
that situation, RADIUS request MAY be retransmitted to another server
that is known to be part of the same pool.
2.6.2. Head of Line Blocking
When using UDP as a transport for RADIUS, there is no ordering of
packets. If a packet sent by a client is lost, that loss has no
effect on subsequent packets sent by that client.
Unlike UDP, TCP is subject to issues related to Head of Line (HoL)
blocking. This occurs when when a TCP segment is lost and a
subsequent TCP segment arrives out of order. While the RADIUS server
can process RADIUS packets out of order, the semantics of TCP makes
this impossible. This limitation can lower the maximum packet
processing rate of RADIUS/TCP.
2.6.3. Shared Secrets
The use of TLS transport does not change the calculation of security-
related fields (such as the Response-Authenticator) in RADIUS
[RFC2865] or RADIUS Dynamic Authorization [RFC5176]. Calculation of
attributes such as User-Password [RFC2865] or Message-Authenticator
[RFC3579] also does not change.
Clients and servers MUST be able to store and manage shared secrets
based on the key described above, of (IP address, port, transport
protocol).
2.6.4. Malformed Packets and Unknown Clients
The RADIUS specifications ([RFC2865], etc.) say that an
implementation should "silently discard" a packet in a number of
circumstances. This action has no further consequences for UDP
transport, as the "next" packet is completely independent of the
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previous one.
When TCP is used as a transport, decoding the "next" packet on a
connection depends on the proper decoding of the previous packet. As
a result, the behavior with respect to discarded packets has to
change.
Implementations of this specification SHOULD treat the "silently
discard" texts referenced above as "silently discard and close the
connection." That is, the TCP connection MUST be closed if any of
the following circumstances are seen:
* Connection from an unknown client
* Packet where the RADIUS "length" field is less than the minimum
RADIUS packet length
* Packet where the RADIUS "length" field is more than the maximum
RADIUS packet length
* Packet that has an Attribute "length" field has value of zero
or one (0 or 1).
* Packet where the attributes do not exactly fill the packet
* Packet where the Request Authenticator fails validation
(where validation is required).
* Packet where the Response Authenticator fails validation
(where validation is required).
* Packet where the Message-Authenticator attribute fails
validation (when it occurs in a packet).
After applying the above rules, there are still two situations where
the previous specifications allow a packet to be "silently discarded"
on reception:
* Packets with an invalid code field
* Response packets that do not match any outstanding request
In these situations, the TCP connections MAY remain open, or MAY be
closed, as an implementation choice. However, the invalid packet
MUST be silently discarded.
These requirements reduce the possibility for a misbehaving client or
server to wreak havoc on the network.
2.6.5. Limitations of the ID Field
The RADIUS ID field is one octet in size. As a result, any one TCP
connection can have only 256 "in flight" RADIUS packets at a time.
If more than 256 simultaneous "in flight" packets are required,
additional TCP connections will need to be opened. This limitation
is also noted in [RFC3539] Section 2.4.
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An additional limit is the requirement to send a Status-Server packet
over the same TCP connection as is used for normal requests. As
noted in [STATUS], the response to a Status-Server packet is either
an Access-Accept or an Accounting-Response. If all IDs were
allocated to normal requests, then there would be no free ID to use
for the Status-Server packet, and it could not be sent over the
connection.
Implementations SHOULD reserve ID zero (0) on each TCP connection for
Status-Server packets. This value was picked arbitrarily, as there
is no reason to choose any one value over another for this use.
Implementors may be tempted to extend RADIUS to permit more than 256
outstanding packets on one connection. However, doing so is a
violation of a fundamental part of the protocol and MUST NOT be done.
Making that extension here is outside of the scope of this
specification.
2.6.6. EAP Sessions
When RADIUS clients send EAP requests using RADIUS/TCP, they SHOULD
choose the same TCP connection for all packets related to one EAP
session. This practice ensures that EAP packets are transmitted in
order, and that problems with any one TCP connection do affect the
minimum number of EAP sessions.
A simple method that may work in many situations is to hash the
contents of the Calling-Station-Id attribute, which normally contains
the MAC address. The output of that hash can be used to select a
particular TCP connection.
However, EAP packets for one EAP session can still be transported
from client to server over multiple paths. Therefore, when a server
receives a RADIUS request containing an EAP request, it MUST be
processed without considering the transport protocol. For TCP
transport, it MUST be processed without considering the source port.
The algorithm suggested in [RFC5080] Section 2.1.1 SHOULD be used to
track EAP sessions, as it is independent of source port and transport
protocol.
The retransmission requirements of Section 2.6.1, above, MUST be
applied to RADIUS encapsulated EAP packets. That is, EAP
retransmissions MUST NOT result in retransmissions of RADIUS packets
over a particular TCP connection. EAP retransmissions MAY result in
retransmission of RADIUS packets over a different TCP connection, but
only when the previous TCP connection is marked DOWN.
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Implementors should be aware that programming a robust TCP
application can be very different from programming a robust UDP
application. It is RECOMMENDED that implementors of this
specification familiarize themselves with TCP application programming
concepts.
Clients and servers SHOULD implement configurable connection limits.
Clients and servers SHOULD implement configurable rate limiting on
new connections. Allowing an unbounded number or rate of TCP
connections may result in resource exhaustion.
Further discussion of implementation issues is outside of the scope
of this document.
3. Diameter Considerations
This document defines TCP as a transport layer for RADIUS. It
defines no new RADIUS attributes or codes. The only interaction with
Diameter is in a RADIUS to Diameter, or in a Diameter to RADIUS
gateway. The RADIUS side of such a gateway MAY implement RADIUS/TCP,
but this change has no effect on Diameter.
4. IANA Considerations
This document requires no action by IANA.
5. Security Considerations
As the RADIUS packet format, signing, and client verification are
unchanged from prior specifications, all of the security issues
outlined in previous specifications for RADIUS/UDP are also
applicable here.
As noted above, clients and servers SHOULD support configurable
connection limits. Allowing an unlimited number of connections may
result in resource exhaustion.
Implementors should consult [RADIUS/TLS] for issues related the
security of RADIUS/TLS, and [RFC5246] for issues related to the
security of the TLS protocol.
Since "bare" TCP does not provide for confidentiality or enable
negotiation of credible ciphersuites, its use is not appropriate for
inter-server communications where strong security is required. As a
result "bare" TCP transport MUST NOT be used without TLS, IPsec, or
other secure upper layer.
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Open issues
Open issues relating to this document are tracked on the following
web site:
http://www.drizzle.com/~aboba/RADEXT/
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